Fibrous media such as paper, cardboard and non-woven materials are used daily as information media, product packaging, technical products, etc. Fibrous media thus form sensitive or valuable information media (bank notes, cheques, identity documents, etc.) and are used in sensitive environments (States, banks, etc.), others are containers of sensitive or high value products (packaging, envelopes etc.), yet others fulfil sensitive technical functions (filtration, storage, etc.).
Methods according to prior art for recognition or authentication of fibrous materials, mostly applicable to paper, usually consist of adding security elements such as physical elements (reactive or non-reactive strips, safety thread, etc.) and/or chemical substances (reagents) and particular manufacturing methods (watermarks, choice of fibres, texturisation, surface treatment, etc.). These security elements imported into/onto the original material make it more difficult to counterfeit the fibrous medium, but cannot be used to recognise each medium individually, and in particular increase the production cost of this type of material very considerably.
Patent application EP 1 202 225 proposes to use the unique physical attributes of a document, so that in a first step a key is generated associated with the document and is encoded in the form of an image itself printed on the document. The authenticity of a document is verified by generating a key in a second step under the same conditions as in the first step, that is encoded in the form of an image, and the image obtained is compared with the image printed on the document. If the image generated during the authentication is identical to the image printed on the document, it is qualified as being authentic, otherwise it is qualified as being non-authentic.
The inventors have observed that under “normal” usage conditions of the method, the document may have been slightly modified during use, for example folded, perforated, printed, etc., after the printed key has been recorded in the form of an image on the document. Furthermore, the document remains “living” during use and environmental variations (humidity, light, etc.) can make its dimensions or colour vary slightly and consequently cause slight variations to the generated key and the associated image.
Furthermore, extraction conditions of the key are difficult to reproduce exactly, for example if different acquisition or illumination equipments are used.
Consequently, this authentication method that requires an identical comparison cannot be considered to be satisfactory, considering that it systematically eliminates any document not having the same image as that obtained during the authenticity verification, without taking account of problems related to the life of the document, or the reproducibility of measurements.